Cables, used for shipping crafts and offshore structures at an early stage, was used without any difficulty if they have predetermined levels of flexibility and durability in combination with electrical properties in use environments. However, many offshore structures have been installed for oil well drilling as an amount of used crude petroleum increases in recent years. Such offshore structures were installed mainly in the sea near the coast, but have been located in the see remote from hot regions such as Africa in recent years. Also, structures have been generally installed in isolated severe cold regions, for example the structures have been installed and operated in very low temperature regions such as near coastal regions of Siberia, Russia.
The cables used in such severe environments should essentially have suitable characteristics for the use environments, and standard requirements according to the conventional standards have been reinforced as uses of the cables increase in the severe environments. For example, there was required a cable having a cold resistance at −15˜−30° C. in prior art, but there has been required a cable having a cold resistance at a temperature of −40° C. or below in recent years in order to meet specific test standards. In addition, there have been many difficult attempts to develop materials and cables that meet desired characteristics including specific standards in addition to the conventional standards.
In recent years, since the offshore structures are installed and operated in the isolated severe cold regions as described above, the offshore structures should be equipped with means capable of solving problems by themselves in an emergency, particularly when a fire breaks out. Accordingly, there has been required a cable having a high flame retardancy, which has been used for the offshore structures in recent years, in order to ensure stability of the cable on firing.
Also, conventional flame-retardant cables may satisfy other desired characteristics and easily ensure flame retardancy by using various rubbers and polymeric resins containing halogen as a halogen content of the cables is regulated to 18% or less. For example, the halogen-containing rubbers such as polychloroprene or chlorosulfonated polyethylene have been used to develop suitable materials and cables for their special purposes. However, these halogen components emit many toxic gases on firing, which cause a loss of lives, as well as do damage to enormous properties, for example corroding expensive equipments used in the offshore structures. In recent years, there have, therefore, been increasing attempts to develop a technique for inhibiting emission of corrosive gases, particularly halogen gas during the combustion by basically regulating a halogen content to 5% or less according to IEC754-1. In addition, the halogen-containing materials are deteriorated in an oil resistance against specific oils such as gasoline or oil containing aliphatic compounds as a main component, and therefore there is a limit to their uses.
There have been attempts to solve basic problems on the halogen content in the prior art by employing non-halogen materials, but the non-halogen materials may not be used for specific applications since they are so expensive and it is difficult to meet desired characteristics except for the basic standards.
The oil resistance of an ocean cable was required to just satisfy durability against specific gasoline components or general water based muds in the prior art, but wide long-term durabilities against specific water based muds such as ester based mud, oil based mud, cement slurry, synthetic oil based mud and the like have been required in recent years.
As described above, there have been attempts to develop a cable that satisfies various standards by developing novel materials that satisfy oil resistance, cold resistance, low toxicity and flame retardancy together. However, there have been many technical limits that a cable meets low toxicity and cold resistance in addition to the basic properties such as oil resistance and flame retardancy, etc., and particularly it was increasingly difficult to ensure a cable covering material that satisfies a cold resistance at −40° C. or below, an oxygen index of 30 or more and IEC 60332-3 Cat.A for flame retardancy together due to conflicting properties of the flame retardancy and the cold resistance.